In general, in a compression refrigeration cycle for refrigerators; e.g., a compression refrigerator having a compressor, a condenser, an expansion valve, and an evaporator, a fluid mixture of a refrigerant and a lubricating oil is circulated in a closed system. Hitherto, in such a compression refrigerator, chlorofluorocarbon such as dichlorodifluoromethane (R-12) or chlorodifluoromethane (R-22) is employed as a refrigerant, and a variety of lubricating oils have been produced and employed in combination with such a refrigerant. Thus, when the aforementioned chlorofluoro compounds that have heretofore been employed as refrigerant are released in air, the ozonosphere in the stratosphere may be depleted, resulting in environmental pollution. Therefore, recently, use of the Flon compound is more and more rigorously controlled throughout the world. Under such circumstances, hydrofluorocarbons and fluorocarbons such as 1,1,1,2-tetrafluoroethane (R-134a) have become of interest as new refrigerants. Although these hydrofluorocarbons and similar compounds are less destructive to the ozonosphere, they have a long life in the atmosphere, possibly resulting in global warming. Therefore, in recent years, there has been investigated use of naturally occurring refrigerant that does not raise the aforementioned problems.
Carbon dioxide is an excellent candidate, since it is harmless to the environment and is safe to human beings. In addition, carbon dioxide has advantages such as easy availability at any place and considerably low cost without necessity for recovery. By virtue of being harmless to the global environment, being free of flammability, and having low toxicity, naturally occurring carbon dioxide coolant has attracted attention in recent years. Possible applications of carbon dioxide refrigerants include electric air conditioners for automobile use, heating apparatuses for use in cold areas, and hot-water supplying systems.
Among these applications, hot-water supplying apparatus will be further described in terms of further enhancement of energy conservation and efficiency, which is demanded in relation to global environmental issues. One advantage of carbon dioxide is that, when carbon dioxide is employed in a heat-pump hot-water supplier, running cost of the supplier is reduced to about ⅕ and coefficient of performance (COP) is enhanced to 3.0 or higher, as compared with gas-system hot-water suppliers generally employed as domestic hot-water suppliers. In contrast, when the aforementioned HFC coolant is employed in heat-pump hot-water suppliers, the maximum temperature of water supplied by the suppliers is limited to about 60° C., because of thermal properties of the coolant. In this case, a compressor of considerably higher output must be further employed. However, when carbon dioxide is employed as a coolant, hot water at about 90° C. can be supplied by virtue of the thermal properties of carbon dioxide. Thus, employment of carbon dioxide is advantageous.
Meanwhile, refrigerator oil is used in a closed-type electric compressor and plays roles including lubrication, sealing, cooling, etc. of sliding parts. However, use of carbon dioxide as a refrigerant often raises the problem that a system employing carbon dioxide therein requires higher discharge pressure and has a higher temperature, as compared with the case where R-134a or the like is employed. As a result, the refrigerator oil in the system is exposed to carbon dioxide under supercritical conditions. Therefore, if a conventionally used lubricating oil is employed for lubrication, unexpected problems arise. For example, the lubricating oil becomes less stable, failing to ensure long-term stable use, and lubrication performance such as wear resistance becomes poor.
Conventionally, synthetic oils having miscibility with refrigerant (e.g., polyalkylene glycol and polyol ester) are generally employed refrigerator oils for use in a refrigeration cycle employing carbon dioxide or a compressor.
Specifically, Japanese Patent Application Laid-Open (kokai) Nos. 10-46169 and 2001-153476 disclose a lubricating oil composition for refrigerators, which composition comprises at least one species selected from polyalkylene glycol and polyvinyl ether and which composition has a kinematic viscosity of at least 5 mm2/s at 100° C., a refrigerating cycle employing the oil composition, and a compressor employing the oil composition. Japanese Patent Application Laid-Open (kokai) Nos. 2000-273477 and 2001-19987 disclose a refrigerator oil composition employing polyester-polyol.
However, some polyalkylene glycols have poor insulating performance attributed to the molecular structure thereof. Since refrigerating oils employed in a closed-type electric compressor are required to serve as electrically insulating oil, when such a polyalkylene glycol species is employed, a short circuit may occur between hermetic terminals provided so as to supply external electric power to a motor of the compressor. Furthermore, high dielectric constant and dielectric tangent result in large leakage current, which may cause accidents by an electric shock.
Polyester-polyol, having excessively high miscibility with carbon dioxide refrigerant, considerably lowers viscosity when it is in a compressor and forms a solution with carbon dioxide, impairing sealing performance on the high-pressure side. In this case, compressing efficiency lowers, and flow of the refrigerating oil into the refrigeration cycle increases, possibly causing a drop in heat exchanging efficiency.
Under such circumstances, an object of the present invention is to provide a refrigerator oil composition for carbon dioxide refrigerant, of which composition is employed in a refrigerating system operated in a supercritical state of carbon dioxide refrigerant and, particularly, to provide such a composition which can enhance heat-exchange efficiency during passage of supercritical carbon dioxide through a heat exchanger as well as which has excellent durability and lubrication performance.